Proliferating cell nuclear antigen (PCNA) for both DNA replication and repair. Most prominent among its necessary functions is to stimulate the processivity and thereby the activity of DNA polymerase delta (pol delta). The objective of this research is to establish the role of PCNA in human mutagenesis is based upon detailed understanding of the mechanisms by which it stimulates pol delta. Since complex formation between human PCNA and pol delta is absolutely DNA-dependent, the hypothesis that complex formation differs with lesion-containing DNA will be tested. Similar tests will be performed for interactions of PCNA and pol delta with other proteins including p21, RF-C and FEN-1. Each represents a functionally (biologically) distinct class of PCNA-interacting macromolecule. Explicit insights will be obtained into the interactions of PCNA-interacting macromolecule. Explicit insights will be obtained functionally (biologically) distinct class of PCNA-interacting, macromolecule. Explicit insights will be obtained into the interactions of PCNA with normal versus damaged DNA, in the presence and absence of other proteins. In the presence of normal (unmutated) PCNA, pol delta is 1-2 orders of magnitude more error-prone (relative to pol delta alone). It was speculated that this, the putative "mutator" effect of PCNA arises from the fundamental mechanism of pol delta-stimulation by PCNA and is hence, unavoidable. This leads to the hypothesis that a group of human mutations is actually secondary, and results from primary or underlying mutations in PCNA. Since these PCNA mutations would not be expected to interfere with either replication or repair, it was further speculated that they would be undiscovered. This proposal outlines plans to test the hypothesis that primary mutations in PCNA lead to secondary mutations of biomedical importance.